Longer:
1) Read http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/ ,
especially http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/ports.html
and http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/cutting-edge.html
(All of the Chapter 4 and all of the Chapter 19. It's not rocket science, but needs
some attention.)
2) Install cvsup and portupgrade.
3) Upgrade the system to RELENG_5_3
(see http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/cvs-tags.html
and don't forget to read /usr/src/UPDATING ).
4) Update ports with cvsup.
5) Read /usr/ports/UPDATING for special cases.
6) Use portupgrade for upgrades of ports. See Chapter 4.5 and man portupgrade for
details.

Best regards,
V.Chukharev

PS. Hardly this is the best place for a question like yours.
PPS. There is no 5.3-CURRENT any more, it's 6-CURRENT now.
sysinstall(1) is meant to deal with its own version only. It is not even updated
from source with the world by default, though I didn't check this lately.
Since 5.0 the ports system changed quite a bit, you really better upgrade the
world for fresh ports.